In recent years, an organic electroluminescence element employing an organic compound(s) (hereinafter also referred to as an “organic EL element” arbitrarily) has been highly expected for its applications to, for example, a large-sized full-colored display element of solid-state light emission available at a low price and to writing light source arrays. Thus, research and development of such an organic electroluminescence element have been actively conducted.
An organic EL element is a thin all-solid-state element composed of a pair of an anode and cathode formed directly or indirectly on a film and an organic functional layer provided between the anode and the cathode, and the organic functional layer which may consists of a single or multiple layers contains an organic light-emitting compound(s) and has a thickness of only about 0.1 μm.
It is known that when a relatively low voltage of about 2 to 20 V is applied to such an organic EL element, electrons are injected from a cathode to an organic compound layer, and electron holes are injected from an anode to the organic compound layer; then these electrons and electron holes are recombined with each other in a light-emitting layer, and energy is released as light upon return of an electron energy level from a conduction band to a valence bond. This technique is expected for use in future flat displays and lighting devices.
In addition, an organic EL element utilizing phosphorescence emission, which has been recently found, can achieve efficiency of light emission of about four times larger in principle than that of a conventional element utilizing fluorescence emission. Thus, research and development regarding layer configurations and electrodes of a light-emitting element utilizing phosphorescence emission, as well as developments of materials for an element utilizing phosphorescence emission, have been extensively conducted all over the world.
In particular, as one of measures for preventing global warming, an organic EL element utilizing phosphorescence emission has begun to be considered to be applied to lighting devices, which currently consumes large part of energy that human consumes. Hence, an organic EL element utilizing phosphorescence emission is extensively studied for improving its efficiency and for decreasing costs for practical realization of a white light-emitting panel, which can be an alternative for conventional lighting devices.
A white light-emitting panel used for lighting is required to have high efficiency and long lifetime. Particularly as to length of lifetime, a white light-emitting panel is inferior to a fluorescent light and white light-emitting LED at present. Thus, various studies for achieving higher efficiency and longer lifetime have been conducted.
Performance of an organic EL element largely depends on thin film morphology. Generally, thin layers in an organic EL element are preferably amorphous.
To provide amorphous thin films, there is a method for preventing crystallization of an organic compound(s) for an organic layer in heating including steps of conducting application of the organic material(s) in an inert gas atmosphere and conducting heating and drying at a temperature 10° C. or more lower than a glass transition temperature Tg of the material in an inert gas atmosphere (see Patent Document 1, for example)
In recent years, it has been recognized as important to control molecular orientation in an amorphous film for controlling electrical and optical properties of an organic EL element. Yokoyama et al. have conducted detail investigations regarding molecular orientation in an amorphous film, and reveal that the molecular orientation largely affects electric charge-transporting properties (see Non-patent Document 1, for example).
Use of flat molecules achieves tight packing, and also improves electrical properties by enhancing interaction between π electrons. However, this method cannot be used for a light-emitting layer generally containing dopant molecules that are not flat.
To improve electrical properties of a light-emitting layer, use of two types of host molecules in combination has been proposed. Improvement in electrical properties is achieved by mixing one type of a host for transporting electron holes with another type of a host for transporting electrons (see Patent Document 2, for example)
This method can easily achieve valance of carriers, whereas in the case of using one type of a host, valance of carriers is difficult to be achieved. However, this method is also difficult to achieve tight packing. Further, considering based on a single carrier, change in packing state may degrade electricity-transporting properties compared to the case of using each of the two types of hosts.
In addition, it has been proved that use of three types of hosts in a light-emitting layer containing non-light-emitting organic materials PVK and OXD-7 improves performances (see Non-Patent Document 2, for example)
In Non-Patent Document 2, large improvement in performances is achieved by simply adding a third type of a host, SimCP2, to obtain a concentration of 8% by mass. Non-patent Document 2 does not describe detail discussions of the reasons of this improvement, but this improvement is probably achieved by confinement effect of excitons by SimCP2 having a large T1. However, further improvements in efficiency and lifetime are desired.
Generally, orientations of molecules in an amorphous film are different, and thus interactions between the molecules are weak. Therefore, an amorphous film has not sufficient electrical properties. In addition, some molecules may be present as fine crystals. In this case, these crystals may function as cores for forming larger crystals in an amorphous film, which causes larger effects of grain boundary and scatters carriers, resulted in shorter lifetime of an element.
Further, fine crystals in a film grow as temperature rises, which makes storage stability in high temperature insufficient.
As described above, an amorphous film has many problems to be solved. In addition, not much is known about a film formed by application of a low-molecular compound(s). Therefore, it has been desired to solve these problems.